Even with the bootstrap tag, you can do the same thing, delete the old tag, add the new one. But I digress.
When I create a "Custom, Magic, and Masterwork" shield, I still have the Shield Bash listed in my weapons tab once I add it to the character. So I'm not sure what you are doing differently than what I am doing. I'm essentially doing what I laid out above.
Bashing, as in the magical enhancement, that would have been good to mention. Got it now. You should be able to set that Shield Bash's damage up one. Here is some simple code as an example.
Code:
container.parent.childexists[wShldBash].field[wDamage].value += 1
This will increase the shield bash up another die. So Bashing would have to have a new version made to replace the old one with something like this scripted. As for how to get it to apply to the Shield Bash, there is some parent child relationship that I don't know right off the top of my head, so I'm not sure I have that 100% right, but it gets you going in the right direction.
And you're right, you can weaponize a shield outside of the spikes, by enchanting the shield as a weapon (in addition to enchanting it separately as a shield). I don't allow it in my game, which is why I was confused. My bad.
Anyhow, the probable reason you are not getting your Shield Bash to work with custom items is that each item is a container, and the only way to access things bootstrapped to a container is to refer to the container's parent or child or gizmo. Yeah, it's about as clear as mud, but that is usually the culprit when things appear on regular items but not on custom items. Unfortunately, I really don't know how you are implementing everything to give you the appropriate steps to rectify the problem.